Don’t be that guy
Exercising the role of repeater trustee brings with it some responsibility of leading by example concerning the proper use of equipment to be heard well by the locals. Such is apparently not the case with my Kenwood TM-D710A and its microphone. Reported symptoms include:
- ~10 Hz ticking noise (there are many other reports on the Internet about this);
- Audio breaking in and out;
- Audio crackling.
These symptoms vary in their severity from zero issues to zero audio and everything in between. Not wanting to be “that guy” on our repeater, I now tend to lay low whilst figuring out the cause of the audio problem.
Over time I have gathered the following additional observations:
- Stretching the cord seems to aggravate the above three conditions;
- The PTT switch and PTT action seem to work perfectly fine;
- If I lean over towards my radio (it’s underneath the passenger seat) things improve, but there is still occasional symptoms;
- While transmitting I seem to be able to aggravate the audio symptoms by varying pressure and placement of my thumb on the PTT.
RJ45: The devil’s microphone connector?
Maybe I am completely wrong, but I always assumed the RJ45 connector is viable only in pristine environments where cables pretty much never move or at least not often, have little to no tension and so on. The little connector on the cord between a desk telephone and the handset is one notable exception, but even then the thin handset wire tends to have much lower tension than our radio mic cords.
My stretching of my D610’s microphone cable always left me with a bit of anguish knowing I am putting tension and side forces on the RJ45 connection on the radio. To help eliminate this possibility I built a platform next to the microphone jack on the TM-D610A and secured the microphone cable to the platform. Now the RJ45 remains fully seated and no longer has sideways forces placed upon it.
My hope for a cure was dashed as my next test and subsequent reports from friends suggest all the symptoms continue to persist. Sigh.
Let’s peek inside
I took the advice of my radio friends and examined the microphone. One obvious observation is if the audio signal goes through the switch, a bad switch may well explain my intermittent audio issues. I opened the microphone and took the following photos…
The stock microphone for the Kenwood TM-D710A comes apart neatly and orderly. The PTT button remains in a rubberized enclosure in the right portion.
This and the next picture confirm the audio signal path does not pass through the PTT switch which is a big relief. The actual switch the PTT button actuates is a simple four-pin (3 ground, 1 hot) device (lower right on the circuit board shown above) and appears to ground the signal on pin 4 (PTT) to pin 5 (GND) when PTT is asserted; Measurements confirm the switch is normally open.
In the photo above, you can see the DTMF board (blue) along with its cable to the main board (brown).
RJ45: In the handset as well
Then, of course, there is this…
Another RJ45 connector. This surprised me. Not shown in the photos is a rubber boot that wraps around the lip you see above providing some measure of weatherproofing. However, nothing actually physically prevents the RJ45 from wiggling a bit during high tension moments despite the connector shape being molded into the rubber boot.
Could this be the primary culprit? Hard to say at this point, but I did clean all the RJ45 contacts here, in the radio and on the microphone cable connectors.
I will update here as I learn more.
Signal paths on the circuit board
I took a good long look at the circuit design of the main microphone circuit board and made this schematic to highlight the audio signal chain…
This does not document everything in the mic such as the PTT switch and signal. Key takeaways include:
- The microphone is biased with a voltage divider powered by the a 5 volt regulator.
- The microphone bias return happens nowhere on this board and instead relies on a connection between the GND and MIC-GND somewhere outside of the microphone. I assume this is within the radio.
- Interestingly, that capacitor next to the 1.5k resistor is connected to the local “power” ground reference.
- The red line highlights the data line to the DTMF circuitry. The only reason I show it here is it runs directly underneath that zero ohm shunt resistor just before the audio leaves the mic via the MIC pin 6. Of course these signals run along together in the mic cable so I guess this isn’t worth worrying about. Interesting though.
Lots of test points abound along with unpopulated component locations like the Do-Not-Install (DNI) resistor above. There is also a spot for an additional regulator – not used in this example. The four wires connecting this board to the DTMF board have two wires for power, but both are shorted together via a zero ohm shunt to the 5 volt regulator. It seems clear this board has life in many different designs.
Also interesting is the way Kenwood handles connecting the metal microphone clip to this board. There is a spring between the clip and an open pad on the board (marked PLATE) connecting the clip to the local “power” ground. In addition a large metal tang, viewable in photos above, presses against yet another isolated pad (marked KNOB) that ties to pin 7 through a series capacitor.
Know thy mic
All this “research” may be wandering in the weeds, but it usually doesn’t hurt to know a bit more about the equipment you rely on. Until I figure out what is really going on, every bit of knowledge is worth having.
We shall see.